Chemical Functionalities at the Silica Surface Determining its Reactivity when Inhaled. Formation and Reactivity of Surface Radicals

Reactive radicals at the surface of quartz or other SiO2 poly- morphs have been studied by EPR in relation to their possible role in pathogenicity. All the examined dusts bear the character- istic radicals of silica ground in air: Si, SiO, SiO2 (peroxyradi- cal) and O2 (superoxide ion), but some also show additional spectral lines belonging to other radical forms. Comparison of standard quartz dusts (DQ-12, Min-u-sil 5) with a natural quartz and with what obtained by grinding a very pure quartz crystal indicates that to a higher purity corresponds a higher rad- ical population. Cristobalite and vitreous silica exhibit similar spectra, with larger proportion by respect to quartz, of partially reduced oxygen forms. The reactivity of the silica surface to- wards O2 and NaClO aqueous solutions are investigated by ex- amining the modification in the EPR spectra induced by these treatments. A possible mechanism for fibrogenicity is proposed whereby, within the activated macrophage, a catalytic reaction occurs between surface functionalities and macrophage oxygen metabolites. This reaction would trigger the abnormal produc- tion of fibroblast stimulating factors, ending up with silicosis.